课堂探究

　　1．Recently，however，scientists have been finding that long­term use of these fertilizers can cause damage to the land and，even more dangerous，to people's health.

　　然而，最近科学家发现长期使用这些肥料会造成土地受损，甚至更危险的是，会对人们的健康造成危害。

　　剖析：1)本句的时态为现在完成进行时，其构成为：have/has＋been＋doing。

　　They have been building the bridge for two months.

　　两个月来他们一直在建桥。

　　All the Chinese have been working hard to make our country richer and stronger.

　　所有中国人一直都在努力工作，以使我们的国家更加富强。

　　2)现在完成进行时和现在完成时的区别：

　　两者都可以表示"从过去开始一直持续到现在"，但现在完成时在含义上着重表示动作的结果，不强调重复性；现在完成进行时着重表示动作的重复和动作的延续性。不能用于进行时的动词一般也不能用于现在完成进行时。如：

　　He has finished his work and now is having a rest.

　　他已经完成了工作，现在正在休息。

　　I have been working in this company since three years ago.

　　自从三年前以来，我一直在这家公司工作。

　　考点 however副词，意思是"然而"，常作插入语，可位于句首、句中或句末。如：

　　It's raining hard.However，I think we should go to school on time.

　　雨下得很大，但我认为该按时上学。

　　On arriving home，however，he found his wife lying on the ground.

　　然而，他一到家，发现妻子躺在地上。

　　The students thought they had done everything as the teacher did.They were mistaken，however.

　　学生们认为他们按老师做的那样做了每件事，然而他们错了。

　　单项填空

　　1)Tom ______ in the library every night over the last three months.

　　A．works　　　　　　　　 B．worked

　　C．has been working D．had been working

　　2)In the last few years thousands of films ______ all over the world.

　　A．have produced B．have been produced

C．are producing D．are being produced